# Building Block Identity Versioning Policy

## Policy
- Semantic Versioning (`MAJOR.MINOR.PATCH`) is mandatory.
- Backward-compatible contract additions increment `MINOR`.
- Breaking schema changes increment `MAJOR`.
- Non-functional clarifications increment `PATCH`.

## Compatibility Guarantees
- Existing required fields are not removed in the same major line.
- Field renames require additive migration period before removal.
- Deprecations must include replacement guidance.

## Release Governance
- Changes require Thalos owner approval.
- Each release includes a consumer impact note.
- Contract ownership statement must remain explicit in release notes.

## Migration Rules
- Introduce additive fields first.
- Keep dual-shape support for one compatibility window.
- Remove deprecated fields only in next major release.
